In mine props of this type the top ram comprises substantially a cylindrical sealed relative to the inner wall of the outer cylindrical tube and which has a prop head at its other end. The top ram further includes an inner prop extension stop and a return spring formed as a screw spring which is secured with one end to the prop head and with the other end to the cylinder foot. The return spring passes through a bore in the piston, whereby a further tube which encloses the spring is arranged coaxially between the prop head and the piston. The piston includes a safety valve which relieves the pressure from the compression chamber when the piston assumes a predetermined end position.
A hydraulic steel mine prop as just described is known from German Utility Model Publication (DE-G) 8,808,519 corresponding to 5,051,039 (Heiliger). Props of this construction do not have an inner mechanical stop which limits the prop extension and takes up the prop extension force. Thus, one version of such props comprise a safety valve in the piston, which valve is operated by mechanical means in the desired end position for releasing the pressure from the compression chamber so that a further prop extension becomes impossible. The mechanical structure conventionally used for this purpose is, on the one hand relatively expensive, and on the other hand, the known structure permits only a passive switching of the safety valve. This means that the safety valve, due to its internal mechanical features, is operated at a switching point and not at all from the outside in a positive manner.